This invention relates to systems, and particularly cooling systems such as domestic refrigerators or freezers, and fixed or mobile air conditioning systems. It relates to equipment for which exchangers work equally well in natural convection and in forced convection.
Most refrigerator and freezer models operate in natural convection equally well for distribution of cooling inside the cooling compartment or outside this compartment to extract heat from the system. The corresponding natural convection coefficients are very low, between 3 and 5 W/m2.K. To transfer heat, these low coefficients of convection necessitate large exchange surface areas and large temperature differences between the exchanger and air, typically equal to 15 to 20 K. For example, a refrigerator with an average inside temperature of 5xc2x0 C. located in a room at an ambient temperature equal to 25xc2x0 C., giving a temperature difference of 20K, requires cooling cycles with evaporation temperatures of the order of xe2x88x9215 to xe2x88x9220xc2x0 C. in the evaporator inside the compartment and condensation temperatures of 35xc2x0 C. to 40xc2x0 C. in the condenser outside the refrigerator. Therefore, the cycle operates with a temperature difference of 50 to 60K to maintain a useful difference of 20K.
It is known that fans can be used inside the compartment to reduce exchange surface areas, have better control over air circulation and to limit temperature differences between the evaporator and air inside the compartment. Similarly, it is also known that fans can be used to increase the exchange coefficients on the condenser, to ventilate the outside surface of the condenser. More generally, for air conditioning systems, fans are used both on the condenser and on the evaporator.
In all cases, the efficiencies of fan-motor units used for low power electrical fans (typically between 10 and 200 W) are low, typically the efficiency of electric drive motors is 10 to 15%, which introduces two types of consequences.
1. For exchangers with natural convection, the addition of a fan improves exchange coefficients and can reduce the difference between the phase change temperature of the cooling fluid and the average temperature of the air circulating over the exchanger. For the same temperature in the refrigerator, this increases the evaporation temperature, and for the same outside air temperature, it reduces the condensation temperature. This reduction in the difference between the evaporation and condensation temperatures, and therefore the corresponding evaporation and condensation pressures, reduces the consumption of the compressor. But the consumption of the fan is significantly greater than the saving of consumption of the compressor. The total consumption of the system (fans+compressor) is greater than the consumption of a compressor alone operating with natural convection exchangers.
2. For exchangers that already use forced convection such as air conditioning systems, and particularly onboard air conditioning systems, the proportion of the consumption of fans in the total energy consumption (compressor+fans) is very significant. For example, in automobile air conditioning, it can be as high as 35% of the total consumption.
The purpose of this invention is to improve the total energy balance (compressor+fans) of refrigeration or air conditioning systems for exchangers with natural convection or forced convection.
More particularly, this invention relates to systems using a fluid with a thermodynamic cycle comprising an expansion phase performed using a pressure reducer and a compression phase achieved using a compressor.
According to the invention, in this type of system, mechanical energy added to the two-phase circulating gas or liquid flow during the compression phase, can be recovered during a liquid-vapour expansion phase.
According to the invention, the liquid-vapour expansion phase is achieved through a two-phase turbine driving a fan causing air circulation over the said exchanger.
Preferably, in the case of a system using a sequence of devices comprising a compressor, a first exchanger acting as a condenser, a second exchanger acting as an evaporator, the system comprises a liquid-vapour expansion phase achieved using a two-phase turbine operating as a pressure reducer, activating a fan causing air circulation over the evaporator or over the condenser. Thus, mechanical energy recovered by replacing the usual pressure reducer in the form of an orifice or a capillary, by a two-phase expansion turbine operating as a pressure reducer comprising mobile elements and directly driving a fan wheel. The passage through the pressure reducer while performing external work also improves the global efficiency of the cycle.
Another preferred embodiment consists of a system using a sequence of devices comprising a compressor, a first exchanger acting as a condenser and a second exchanger acting as an evaporator, and this system comprises:
a first turbine activating a first fan driving air circulation over the condenser,
a second two-phase turbine acting as a pressure reducer and driving a second fan forcing air circulation over the evaporator.
The fan wheel is located either in the compartment to be cooled like the compartment in a refrigerator or a freezer, or outside it for example to ventilate exchange surfaces of the condenser of a refrigerator, or the heat exchange surfaces in air conditioning systems. In all cases, the two-phase pressure reduction turbine is located such that it can directly drive a fan wheel transferring the air flow over the exchange surface.
Advantageously, the system according to the invention is such that:
the spindle of the turbine controlling the two-phase pressure reduction of the cooling fluid passes through the turbine body,
a seal is provided at the joint between the turbine spindle and the body of the turbine to make it leak tight.
Also advantageously, in the case of another embodiment, the turbine controlling the two-phase pressure reduction of the cooling fluid drives the fan through a magnetic drive. This solves the problem of the seal between the turbine body and the turbine spindle.
In a process for the production of cold or heat using a cooling fluid with a thermodynamic cycle comprising at least one liquid-vapour expansion phase, the invention also relates to the characteristic step consisting of achieving the said liquid-vapour pressure reduction by means of a two-phase turbine activating a fan driving air circulation over an exchanger.
Other characteristics and advantages will become clear after reading the description of variant embodiments of the invention given for guidance, and that are in no way limitative.